1. Field of the Invention
The present invention relates to a pneumatic impact pulverizer using high-pressure gas in the form of a jet stream, a fine powder production apparatus having a pneumatic classifying means and a pneumatic impact pulverizing means designed for pulverization using high-pressure gas, and a process for producing toner for developing electrostatic images.
2. Related Background Art
A pneumatic impact pulverizer using high-pressure gas in the form of a jet stream carriers raw powder material with the jet stream, and ejects the raw material from the outlet of an accelerating tube so that the raw material will collide against the impact surface of an impact member that is opposed to the opening plane of the outlet of the accelerating tube. This induces impact force and thereby pulverizes the raw powder material.
For example, in a pneumatic impact pulverizer shown in FIG. 23, an impact member 43 is opposed to an outlet 45 of an accelerating tube 46 to which a high-pressure gas feed nozzle 47 is connected. High-pressure gas supplied to the accelerating tube 46 attracts raw powder material into the accelerating tube 46 through a raw powder material feed port formed in the middle of the accelerating tube 46. Then, the raw powder material is ejected together with the high-pressure gas to collide with an impact surface of the impact member 43. The impact pulverizes the raw powder material.
In the pneumatic impact pulverizer shown in FIG. 23, a pulverization powder feed port 40 is formed in the middle of the accelerating tube 46. Therefore, the powder to be pulverized that has been attracted to the accelerating tube 46 rapidly changes its route towards the outlet of the accelerating tube due to a high-pressure air current ejected through a high-pressure gas supply nozzle 47 immediately after passing through the pulverization powder feed port 40. While changing the route, the powder to be pulverized is dispersed in the high-pressure air current and accelerated quickly. In this state, relatively coarse particles of the powder to be pulverized are involved in the portion of the high-pressure air current that is flowing at a lower flow velocity in the accelerating tube, because of the influence of inertial force. Relatively fine particles are involved in the portion of the high-pressure air current flow that is flowing at a higher flow velocity in the accelerating tube. Thus, the particles are not dispersed uniformly within the high-pressure air current. Therefore, the high-pressure current remains separated into a flow having higher concentration of power to be pulverized and a flow having lower concentration of powder to be pulverized. Then, when the high-pressure air current collides with an opposed impact member together with the powder to be pulverized, the powder to be pulverized concentrates on part of the impact member. This deteriorates pulverization efficiency and degrades throughput.
In the vicinity of an impact surface 41, dust concentration is likely to increase because of the presence of powder to be pulverized and pulverized powder. If the powder to be pulverized contains a resin or other material having a low fusion point, the powder to be pulverized may fuse, become coarser, and coagulate. If the powder to be pulverized is abrasive, the impact surface of an impact member or the accelerating tube may suffer from powder abrasion. This results in frequent replacement of the impact member. There remain some problems that must be overcome to ensure continuous stable production.
Japanese Patent Application Laid-Open No. 1-254266 has proposed a pulverizer in which the tip of an impact surface of an impact member has a conical shape with an apex angle of 110.degree. to 175.degree.. Japanese Patent Application Laid-Open No. 1-148740 has described a pulverizer whose impact surface is formed as an impact plate having a projection on a plane perpendicular to an extension of the center axis of an impact member. These pulverizers successfully suppresses a localized rise of dust concentration in the vicinity of the impact surface. Therefore, pulverized powder is less likely to fuse, become coarser, and coagulate. Pulverization efficiency has improved, out a more significant breakthrough is desirable.
A variety of pneumatic classifiers have been proposed in the past. These pneumatic classifiers are combined with pneumatic impact pulverizers to form fine powder production systems. A typical system is, as shown in FIG. 24, a dispersion separator (manufactured by Japan Pneumatic Industries Co., Ltd. ).
A powder material feeder for feeding powder to a classifying chamber 64 of the foregoing pneumatic classifier shown in FIG. 24 is shaped like a cyclone. A guide chamber 62 is resting upright on the center of the top of an upper cover 70. A feed pipe 63 is connected to the outer circumferential surface of the upper part of the guide chamber 62. The feed pipe 63 is connected in such a manner that supplied powder will head for the circumferential tangent of the guide chamber.
In the pneumatic classifier shown in FIG. 24, a classifying louver 65 is arranged in the circumferential direction in the lower part of a body casing 71. Classification air that brings a whirling stream from outside to the classifying chamber 64 enters through the classifying louver 65.
A conical (bevel) classifying plate 67 having its center swelled is installed on the bottom of the classifying chamber 64. As coarse powder discharge opening 66 is formed along the outer circumference of the classifying plate 67. A fine powder discharge chute 68 is connected to the center of the classifying plate 67. The lower end of the fine powder discharge chute 68 is bent in the shape of an L. The bending end portion is located outside the side wall of the lower casing 72. The fine powder discharge chute 68 is connected to a suction fan via a cyclone, dust collector, or other fine powder collecting means. The suction fan induces suction force in the classifying chamber 64. With the suction force, suction air entering the classifying chamber 64 via the apertures of the louver 65 develops a whirling stream required for classification.
On feeding powder material to the guide chamber 62 through the feed pipe 63, the powder material whirls down on the inner circumferential surface of the guide chamber 62. Since the powder material descends in the form of a band from the feed pipe 63 along the inner circumferential surface of the guide chamber 62, distribution and concentration of powder material entering the classifying chamber 64 is not uniform (because powder material enters the classifying chamber while flowing on part of the inner circumferential surface of a guide cylinder). Poor dispersion ensues.
Higher throughput tends to result in further coagulation of powder material and insufficient dispersion. This cripples high-precision classification. When an amount of air for carrying powder material is large, enormous air flows into the classifying chamber. Accordingly, the center-oriented velocities of whirling particles in the chamber increase. Consequently, the diameters of separated particles become larger.
Therefore, in efforts to reduce the diameter of a separated particle, a damper 61 is usually placed on the top of the guide chamber to control an amount of air. When a quantity of deaeration is large, part of powder material is discharged and, therefore, lost.
In recent years, copying machines and printers have been required to offer higher image quality and precision. With this trend, required performance of toner serving as a developer has been evaluated more severely. Particles of toner become smaller. There is a demand for toner showing a sharp distribution of particle sizes; that is, a distribution of particles including no coarse particles and less very fine particles.
According to a general process of producing toner for developing electrostatic images, various colorants for producing toner colors, a charge control agent for applying electric charges to toner particles, in a single-component developing method disclosed in Japanese Patent Laid-Open Nos. 54-42141 and 55-18656, various magnetic materials for improving the capability of toner of being carried, and, if necessary, a parting agent and a fluidity facilitator are mixed in a dry process. Using a rolling-mill, extruder, or other kneader, the mixture is melted and kneaded. Then, the kneaded mixture is cooled and caked. Then, a jet stream pulverizer, a mechanical impact pulverizer, or other pulverizer is used to pulverize the caked mixture. A pneumatic classifier is used to classify the pulverized powder. Thus, the particles of the powder are down-sized to have a weight-average particle diameter of 3 to 20 .mu.m that is suitable for toner. Then, if necessary, a fluidity agent or a lubricant is mixed to complete toner. For a double-component developing method, the toner is mixed with various magnetic carriers and supplied for image formation.
As described above, fine toner particles have been produced wholly or partly using the process represented as the flow chart of FIG. 25.
Coarsely-pulverized toner powder is fed continuously or sequentially to a first classifying means, and classified. Coarse powder composed mainly of coarse particles that are larger than a specified size is fed to a pulverizing means, and pulverized. Then, the pulverized powder is fed back to the first classifying means.
A finely-pulverized toner product composed mainly of other particles within or smaller than the specified size is fed to a second classifying means and classified into middle-sized powder composed mainly of particles having the specified size and fine powder composed mainly of particles smaller than the specified size.
Various pulverizers can be employed as the pulverizing means. When coarsely-pulverized powder whose main component is a binder resin is concerned, a jet stream pulverizer using a jet stream shown in FIG. 23, especially, a pneumatic impact pulverizer is employed. As described previously, the pulverizer shown in FIG. 23 offers poor pulverization efficiency and low throughput.
A classifier used as the first classifying means may be a rotor classifier in which classifying brades rotate to develop a whirling stream forcibly and thus performs classification, or a spiral pneumatic classifier that uses an air current taken in from outside to produce a whirling stream and thus performs classification. For classifying toner whose main component is a binder resin, the spiral pneumatic classifier is preferred because of its design in which a smaller movable section is brought into contact with powder.
As described previously, powder material (toner powder) comes out of a feed pipe 63 and descends in the form of a band along the inner circumferential surface of a guide cylinder 62. Powder material (toner powder) entering a classifying chamber 64 is not uniform in distribution and concentration. The powder material (toner powder) flows only along part of the inner circumferential surface of a guide cylinder and flows into the classifying chamber. Therefore, the powder material disperses poorly. When throughput is enhanced, powder material tends to coagulate more frequently and disperses insufficiently. Classification precision deteriorates. A finely-pulverized toner product fails to provide sharp distribution of particle sizes. The distribution becomes broad, the toner quality degrades, and the yield decreases.